1. Field of the Invention
The invention is generally directed towards diagnostic image processing, and, more particularly to a device and system for position and motion sensing of a spherical object surface, and to a method for detecting such motion and registering a series of temporal image frames of the object surface.
2. Description of Related Art
The first step in any ophthalmic procedure is to diagnose the pathology of the eye and/or characterize its structure. In the field of refractive surgery for correcting defective vision of a person""s eye, it is important to gather accurate information about the topography and pachymetry of the eye as well as the nature and magnitude of the aberrations that cause poor vision. The gathering of this diagnostic information typically requires the patient to engage some type of instrument and steadily view a target while the diagnostic information is acquired, usually through multiple video images taken over a time interval ranging from fractions of-to several seconds. This is a relatively long time frame in relation to the saccadic movement of the eye and the time it takes for a person to change their gaze during a measurement sequence. Voluntary or involuntary eye motion may cause a particular measurement parameter to be inaccurately recorded and a resulting vision correction application to be less than optimal.
In light of this, the inventors have recognized a need for a way to compensate for eye motion during the diagnostic procedure. Although the invention will be described with reference to monitoring and compensating for motion of a patient""s eye, it will be appreciated that the apparatus and methods disclosed herein will apply to the monitoring and compensating of motion of any object having a spherical or quasi spherical object surface.
An embodiment of the invention is directed to a device for monitoring the spatial position of, and tracking the movement of, an object having a spherical or quasi spherical surface. The preferable object is the cornea of a person""s eye. The device includes a projection component that employs at least one light-emitting element for projecting at least four spots of light onto the surface where at least two of the spots on the surface must not lie in the same plane. In addition, an image capture component captures a series of real-time video frames of the image of the object surface and the light spots illuminated on the surface. A computing component operates in conjunction with the image capture component and receives the images from the image capture component, locates each illumination light spot in each image frame, and determines a relationship between the image spots in each image frame. The device will preferably have four light-emitting elements that project four corresponding light spots onto the surface; and more preferably will include six light-emitting elements that illuminate portions of the surface with six corresponding light spots. According to a preferred aspect of a device embodiment of the invention, a banking/switching component is used in conjunction with six light-emitting elements and allows four of the six light-emitting elements to form light spots on the corneal surface at any given instant. This is useful when a competing light source is used in a particular application, for example, a scanned slit of light to measure corneal pachymetry. Applications that do not use a competing light source, for example, optical coherence tomography (OCT) and B/C scan require only four light spots. The type of light source is not critical and may include sources such as laser diodes, super luminescent diodes with associated focussing optics, or an incandescent light source with an optical accompaniment that will allow the light source to be focussed on the surface of the object, for example. The light sources are positioned in a planar arrangement with the plane normal to a reference axis of the object surface which, for the purpose of clarity of description, is preferably the optical axis of the cornea of the eye being monitored. Each of the light sources is aimed at the cornea of the subject eye in a pattern that is anywhere from about xc2xd to ⅔ of the distance from the center of the eye to the limbal boundary. This projection pattern allows the center of the cornea to remain free of light scatter which could be important if one wants to view the center of the cornea or view through the iris of the subject eye. Each of the light beams forming a light spot on the object surface diffusely scatters when striking the cornea due to the fragmented cell structure of the cornea. This scatter is seen as small, diffuse dots of light on the surface of the cornea. The relationship of the spots to one another is imaged, and then captured during each video frame of the diagnostic procedure. It is this relationship of the spots that allows the motion of the eye to be tracked in space.
The center of the sphere in space can be mathematically determined by four non-collinear points if each is defined in all three axes, x, y, and z. These four points will define the surface of the sphere. If a sphere can be determined by the location of these points, and four points of light are provided and compared to a reference, then the relative location of the sphere can be determined. Thus, a control image is established according to the invention for such a comparison. When the image of the sphere with the four incident light beams is captured by the imaging device and the resultant image is processed to locate the spots, comparison between sequential image spots yields spot location differences, capturing the amplitude and direction of patient eye motion with each frame. As each image is captured and the spots identified, a compensation vector for each image in the temporal sequence of image frames is constructed. If the object moves forward, between images, the spots in the subsequent image will be spread further apart. If the object moves up or down, or left or right, the spots will move asymmetrically in relation to the motion.
After the image capture system captures these images, they are sent to a computing device. The computing device locates each spot on each image. It then compares what the motion of each of the spots is in each frame and allows the images to be scaled, rotated, and moved in order to be reassembled accurately. The compensation vector is constructed and utilized to logically shift each frame into a uniform central frame of reference thus compensating for any motion of the source during the video image capture process. The computing device reassembles the images while compensating for the movement changes and allows the images to be re-registered with respect to one another with each spherical surface being concentric on the center. This allows motion compensation along all three axis.
The invention provides distinct advantages over a collinear tracking system. For example, the compensation vector is quick to generate once the light scatter spots are identified on the surface image. The methodology according to the invention is unique, numerical, and easily verified. Since the light spots are not collinear, each image will be unique to the eye that was captured. The invention does not rely on light reflection to indicate motion, thus a poorly reflecting cornea can easily be tracked.
These and other advantages and objects of the present invention will become more readily apparent from the detailed description to follow. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art based upon the description and drawings herein and the appended claims.